The manufacture of agglomerated zeolite adsorbent and catalyst compositions has been as much an art as a science, and much of the manufacturing knowledge is built upon a foundation of clay types or blends (e.g. attapulgite, sepiolite, kaolin, bentonite and so on) as the principal binding agent. A binding agent is necessary to prepare agglomerated zeolite based compositions, since the zeolite crystallites are not sufficiently self-binding. A binding agent therefore is used for the purpose of preparing zeolite agglomerates having adequate strength characteristics including crushing strength and attrition resistance for use in for example classical packed bed adsorption processes, including pressure swing adsorption (PSA), vacuum pressure swing adsorption (VPSA) and temperature swing adsorption (TSA) processes, and other reactive processes. While this invention is primarily directed to adsorbent compositions, the same principles discussed herein are also applicable to zeolite catalysts.
It has long been recognized that agglomerated adsorbents require a certain density to achieve the necessary resistance to crushing in commercial service, as well as sufficient porosity to avoid excessive resistance to transport of the adsorbing fluid components. Such compromise between strength and transport resulted in a relative narrow range of physical properties for agglomerated zeolite adsorbents, and, as recognized in U.S. Pat. No. 6,500,234, a similar narrow range of pore diffusivity. The pore diffusivity is a direct reflection of the complex interconnecting network of pores that make up the void structure within an agglomerated zeolite particle. The performance of an adsorbent in an adsorption application is dependent on several factors including, the adsorption capacity for the more strongly adsorbable gas, the relative selectivity of the gases in the mixture and the adsorption kinetics (resistance to fluid transport). Recently, there has been a drive to improve adsorptive separation performance by increasing the capacity and selectivity and decreasing the transport resistance of agglomerated adsorbent materials. The present invention is aimed at reducing the transport resistance of crystalline microporous solid adsorbents, preferably zeolite based adsorbents, while maintaining adequate crushing strength, adsorptive capacity, and selectivity.
Prior teachings seek to improve adsorptive separation performance by increasing adsorption rate through the formulation and processing of the agglomerated adsorbent material. However, most of this art ascribes adsorption kinetics to a limited (and possibly misleading) description of pore morphology obtained by Hg porosimetry characterization. Hg porosimetry characterization is based upon an assumed straight cylindrical pore geometry and can be misleading when there are large volumes connected by narrow pores (sometimes referred to as “ink bottle pores”). While Hg porosimetry characterization is a useful technique for determining porosity, particle density and effective “cylindrical” pore size distribution, the method provides insufficient information regarding pore length, shape, connectivity or tortuosity. It is clearly evident that the intrinsic pore diffusivity of agglomerated particles is directly dependent upon the geometry of the macropores present within said particles. As used herein, the term “geometry of the macropores” is intended to be equivalent to the terms “void structure,” “pore geometry” and “pore morphology.” This pore geometry is the result of a complex (tortuous) network of interconnecting pores of varying size, shape and length which, as taught in this disclosure, is derived from an elaborate interplay between the size, shape, distribution and amounts of the fundamental adsorbent and binder particles which are combined together by the agglomeration process to form the adsorbent agglomerate. By its very nature such geometry is difficult to define or characterize in detail. Hg porosimetry characterization provides an incomplete picture, i.e. total macropore porosity, cumulative pore volume, mean pore diameter and pore size distribution, and does not provide a complete description of the void structure. This fact has been at least recognized in fundamental studies, e.g. “pore size distributions, however, provide incomplete descriptions of the connectivity among voids, which are essential to describe transport dynamics in porous solids” (Zalc, J. M., et al., “The Effects of Diffusion Mechanism and Void Structure on Transport Rates and Tortuosity Factors in Complex Porous Structures,” Chem. Eng. Sci., 59, 2947-2960, 2004). In addition to the fact that pore size and pore size distributions are referenced to cylindrical pores in Hg porosimetry characterization, these results can also be misleading for some pore networks due to the sequential filling of large to small pores with increasing Hg pressure. Despite the recognition of this complex pore structure, the prior art oversimplifies the full range of porous solid adsorbent characteristics needed to understand and achieve improved transport kinetics.
U.S. Pat. No. 6,171,370 discloses a clay-bound low silica type X (LSX) zeolite with SiO2/Al2O3 from 1.9 to 2.1, “wherein the average pore diameter of the macropores is equal to or larger than the mean free path of an adsorbable component when desorbing the adsorbable component from the adsorbent.” The fractional volume of macropores of such average pore diameter is prescribed to be at least 70% as characterized by Hg porosimetry. The invention is directed to LiX adsorbents for separating air to produce O2. Sepiolite and attapulgite clay binders as “needle crystals” in the form of fibers are preferred to form the macropores.
U.S. patent application (US2011/0105301) discloses a faujasite (type X) zeolite with SiO2/Al2O3 from 2.1 to 2.5, a mean transport diameter >300 nm and a mesopore fraction <10% as characterized by Hg porosimetry. A method of producing a binderless NaX (2.35) zeolite is also disclosed. This method involves the addition of kaolin or meta kaolin clay and subsequent caustic digestion to transform the clay to zeolite. The adsorption capacity of the binderless agglomerate (for CO2, H2O and N2) is equivalent to the raw zeolite powder before adding the binder and subsequent digestion/granulation.
The effect of adsorption rate upon the separation of isomers of aromatic hydrocarbons in the liquid phase using BaX and BaKX zeolites is disclosed in US2011/0105301. The process for making such adsorbents includes the addition of a zeolitizable clay binder such as kaolin. This binder is added in amounts from 5 wt % to 0.12 wt % and subsequently converted to zeolite by caustic digestion. The agglomerated zeolite has a total pore volume of at least 0.26 ml/g wherein at least 60% of this pore volume is within a pore diameter range of 100 nm-500 nm as characterized by Hg porosimetry. Also disclosed is the addition of a combustible pore forming agent or “shaping auxiliary” with subsequent burn out thereof during calcination.
U.S. Pat. No. 6,500,234 discloses a process for the separation of nitrogen from gas mixtures using adsorbents having a mass transfer coefficient for N2 of kN2≧12 s−1 and an intrinsic diffusivity for N2 (when measured at 1.5 bar and 300K)≧3.5×10−6 m2/s. Adsorbents meeting these requirements are disclosed in co-filed U.S. Pat. No. 6,425,940. The improved intrinsic diffusivity of such adsorbents is the result of forming agglomerates with a low amount of clay binder and the subsequent caustic digestion and conversion of that binder to zeolite. Furthermore, the process of making such adsorbents is said to result in the formation of a “trunk” and “tributary” macropore system where the “trunk” pores are in the range of 0.1-1.0 μm and the “tributary” pores are less than 0.1 μm as characterized by Hg porosimetry. U.S. Pat. No. 6,425,940 also discloses caustically digested binderless adsorbents that include pore-directing or pore-forming additives such as Nylon or Rayon fibers or corn starch. Such additives are burned out during calcination to form a bidisperse macropore system resulting in improved gas transport properties.
As shown from the above patents, several attempts have been made to correlate “higher rate characteristics” of adsorbents to parameters derived from Hg porosimetry characterization of the adsorbent or with the Knudsen number (Kn). Such attempts are not believed to be fully instructive and fail either because of incomplete or inaccurate characterization of the pore geometry. Both Kn and Hg porosimetry techniques rely upon defining the pores in terms of simple straight cylinders which is an oversimplification of the pore structure. Furthermore, Kn (which is the ratio of molecule-pore wall collisions to molecule-molecule collisions) depends upon accurate definition and determination of a “characteristic length,” e.g. Kn=λ/l, where λ=mean free path and l=characteristic length. The characteristic length is a complex function of the pore geometry and cannot be simply defined by an average equivalent cylindrical pore diameter determined by Hg porosimetry.
Attempts to define adsorbent materials with low transport resistance are further diluted by the use of gross separation performance as a measure of improved rate effects and fail to accurately incorporate the fact that such processes are nonlinear and that separation performance can be affected by equilibrium and physical (in addition to kinetic) characteristics of the adsorbent. Correlations between rate and pore volume and/or pore diameter of the adsorbent are viewed as inadequate and fail to provide the necessary link between the intrinsic diffusivity of the agglomerated adsorbent and a detailed description of its macropore morphology. In short, the prior art fails to adequately identify useful high rate adsorbent materials and/or reliable and cost-effective methods for producing them.
Intrinsic diffusivity is a property of the porous solid material, correctly reflects the transport dynamics imposed by the void structure, and is a necessary criterion for defining any “high rate” adsorption material. In the absence of an accurate means to characterize the detailed pore geometry and an explicit relationship between this geometry and the transport dynamics, intrinsic diffusivity is the most direct and effective measure of the product of specific ingredients and the method of combining them into an agglomerated adsorbent.
It is noteworthy that the bulk of commercial zeolite adsorbents are clay bound, primarily due to the successful balance achieved between transport and strength. However, for the very reasons that clay binding agents achieve adequate strength characteristics, they are found to be limiting with respect to achieving lower transport resistance. It is further observed that the physical properties (namely shape and size) of clay binders are inherently difficult to control during agglomeration and in post processing treatments of the agglomerate. To further overcome some of these transport limitations of clay based adsorbent formulations, the prior teachings have turned to the use of caustic digestion processes, with and without the use of supplemental pore forming agents, to prepare essentially binderless adsorbents by conversion of zeolitizable clays, to active adsorbents. However, whilst there are examples where pore diffusivities of the adsorbent materials are reported and shown to have been improved, these improvements are by no means guaranteed by virtue of the complex pore geometry discussed above. Moreover, additional steps, time and often equipment are required which increases the manufacturing complexity and adsorbent cost.
The problem remains of how to predictably develop a binder containing agglomerated adsorbent composition, including a method of making the same, to maximize its intrinsic pore diffusivity which in turn will enhance adsorptive separation performance. The present invention solves this problem by first creating and manipulating the pore morphology of the agglomerated adsorbent using developed model constructs of porous materials, relying only upon the physical properties of the adsorbent and binder particles rather than their chemical composition. The diffusion of gas is then simulated in a variety of model porous materials having different void structures to identify parameters of the solid material that minimize transport resistance, i.e. maximize intrinsic diffusivity. The result is a prescription for creating an ideal high rate adsorbent composition. This prescription is then applied within the limitations (both physical and compositional) of the available raw materials to identify and produce commercial zeolite based adsorbent compositions with high intrinsic diffusivity.